I bought Painter 12 a while back and I really like it (and if you change to the Legacy Brush Resize tool, it's fairly stable too, at least for me).
I tried to get familiar with more of the stock brushes this time around, so this review includes some new brushes, but some old brushes I really liked as well. This is a compressed review though, for the full version look here.
Least favorite new brush: Real Oils, any of them
Favorite new brush that has actually been around for a few versions of Painter: "Grainy Water" Blender
What I hated most of all: Photoshop CS5's Mixer Brush Tool
Just so you know what criteria I'm judging by, if a brush is easy to get the behavior I want out of it, and the results look pretty good, then it is a good brush. Brushes that are clumsy or non-intuitive get a low rating, regardless as to how good they look. I want efficiency in painting more than anything else, and if I feel like I'm wrestling the program, my joy in painting just disappears.
So here's some of the other brushes I spent some time with:
Painter 10+ (and maybe earlier versions)
Blenders> Grainy Water: 2 thumbs up, best blender even in Painter 12
Watercolor> Wet Wash Flat: the least offensive of the watercolors for actual painting
Watercolor> Diffuse Bristle: great for adding natural texture
Digital Watercolor> New Simple Watercolor: 2 thumbs up, best for adding color to drawings
Real 2b Pencil: 2 thumbs up. Most useful for drawing.
Variable Chisel Tip Marker: 2 thumbs up when opacity control changed to "Pressure." Corel gets Markers right.
Real Pastels/Chalk: 1 thumb up by themselves, but two thumbs up with Grainy Water Blender
Real Watercolors: 1 thumb down. You can't predict what they are going to do!
Real Oils: 2 thumbs down (nice looking, but painful to use)
Photoshop CS5 (CS5 doesn't have real stock brushes, so this is a review of the brush engine)
Basic brush tool: 1 thumb up if you use a Brush Preset that has some texture and add a little spacing/scatter/texture, 1 1/2 thumbs up when using dual brush functionality
Smudge tool: 2 thumbs down by itself, but if you modify it with a Preset, tie the strength to your pen's pressure, and adjust the spacing/scatter, you can get it to do some nice things.
Mixer Brush Tool: Waste of effort. Photoshop basically copied Painter's "Oils" controls, which are the most unwieldy part of Painter's brush engine.